A heavy crossbow requires two hands to use effectively, regardless of the user's size. Loading a heavy crossbow is a full-round action that provokes attacks of opportunity.

A Medium-size or larger character can shoot, but not load, a heavy crossbow with one hand at a -4 penalty. A Medium-size or larger character can shoot a heavy crossbow with each hand at a -6 penalty, plus the usual -4 penalty for the off-hand attack (-6 primary hand/-10 off hand). The Two-Weapon Fighting feat does not reduce these penalties because it represents skill with melee weapons, not ranged weapons. The Ambidexterity feat lets someone avoid the -4 off-hand penalty (-6 primary hand/-6 off hand).